Ingenuity for finding a personal use for dead space is well regarded here. Kudos.

Many of my touring bikes and tandems have water bottle braze on mounts on the bottom of the down-tube that would place that water bottle in the top half of the area holding the OP's lock-thing. Some others have pegs for old-school air pump down there. I really like having an extra bottle of water and have been often guilty of removing the bottom side pump pegs on bikes that aren't collectible or historically significant.